The present invention relates to electron beam welding.
It is known that the mechanical characteristics, such as the tensile strength, the hardness or the resilience, of a weld, considered in the crude state of solidification, that is to say before any heat treatment, depend on the chemical composition of the molten zone and also on the welding heat cycle to which the weld has been subjected.
As regards the heat cycle, it is a function only of the energy which is actually supplied to the joint between the pieces to be welded, and this applies for a given geometry of joint and when the welding is carried out in a single pass.
The various parameters taken into account for an electron beam welding operation are:
the accelerating voltage of the electron gun (U), PA1 the intensity of the current delivered by the gun (I.sub.f), PA1 the focusing current (I.sub.foc), PA1 the firing distance (d), i.e. the distance between the plane of the focusing coil of the electron beam and the pieces to be welded, and PA1 the welding speed (V). PA1 back-scattered electrons to which the power P.sub.b corresponds, PA1 secondary electrons to which the power P.sub.s corresponds, PA1 electrons of thermal origin, to which the power P.sub.th corresponds, and PA1 electrons transmitted through the pieces, in the case of a projecting weld, to which the power P.sub.tr corresponds.
It is also possible to take into consideration parameters which define possible vibrations of the electron beam.
One of the parameters derived from the above is the firing power of the gun: EQU P.sub.f =U.multidot.I
It is known that only a fraction P.sub.abs of this power will in fact be absorbed by the pieces to be welded; the relative magnitude of P.sub.abs essentially depends on the distribution of the power density, which distribution is in turn a function of the exact position of the focusing point of the electron beam, that is to say a function of the value of the current I.sub.foc. P.sub.abs also depends on the distance (d), but the regulation of P.sub.abs essentially depends on the parameter I.sub.foc, which shows that the magnitude of the latter must be considered as predominant.
As regards the electron beam emitted by the gun, it is known that, in addition to the part of this beam corresponding to the power absorbed, it also gives rise to a certain number of derived electron beams, namely:
The firing power (P.sub.f) is thus expressed as follows: EQU P.sub.f =P.sub.abs +P.sub.b +P.sub.s +P.sub.th +P.sub.tr
The intensities corresponding in particular to P.sub.f, P.sub.abs and P.sub.tr are obtained by dividing the value of each of these powers by the value, which is always the same, of the accelerating voltage U of the gun.
The value of P.sub.abs, which can be derived from this equation, governs the heat cycle of the operation for a given welding speed, for a given steel and for a given geometry of the pieces to be welded.
The value of the fraction P.sub.b can be considered as being between 15% and 25% of P.sub.f, which leaves the sum of P.sub.abs +P.sub.tr with a value of between 75% and 85% of the firing power, neglecting, in a first approximation, the other two factors, namely P.sub.s and P.sub.th, the values of which are low.
It is also known that in order to obtain a good weld in the case of mild or slightly alloyed steels, the power P.sub.abs should be held below a certain limit in order to accelerate the heat cycle as much as possible and hence accordingly to reduce the heating-up of the pieces to be joined, outside the welding zone.
It follows that, in the case envisaged, it is appropriate to assign a minimum value to P.sub.tr, it being necessary for the sum P.sub.abs +P.sub.tr to remain approximately constant.
Having thus determined the various factors and parameters involved in an electron beam welding operation, attention will now be turned more particularly to the operations of this kind in the case where they are required to maintain the morphological and mechanical characteristics of the welds obtained, and also to exhibit a good reproducibility of these same characteristics, from one operation to the next.
Now, experiment proves that, in certain cases, knowledge of the usual parameters (U, I.sub.f, I.sub.foc, d and V) is insufficient to guarantee and adequate invariability and an adequate reproducibility of the said characteristics, and this is because of the significant amplification effect or multiplication factor existing between the values of the said parameters and factors and the values of the desired characteristics; very small variations in the former cause quite unacceptably large variations in the latter.
Thus, for example, the amplification effect is of the order of 50 between the variations in I.sub.foc and the variations in a given mechanical characteristic, namely the HV5 hardness of a steel having the following chemical composition: C 0.14; Mn 1.23; Si 0.22; Ni 0.43; Mo 0.18; B 17 ppm, for a firing power of 22.4 KW, a welding speed of 40 cm/minute, a distance of 600 mm and a thickness of 32 mm for each of the pieces to be welded.
The amplification effect, in the same example, between the variations in I.sub.foc and those in P.sub.abs is 150.
Using the above example as an illustration, it is found that the amplification effect between the variations in P.sub.abs and the variations in the mechanical characteristic envisioned is only 0.33, that is to say that the mechanical characteristic in question varies three times less rapidly than P.sub.abs.
If a certain variation in the value of I.sub.foc gives rise to a substantially greater corresponding variation in the value of the mechanical characteristic in question, it being possible for the latter to be 20 to 50 times greater, for example, the variation in P.sub.abs which corresponds thereto is even greater, namely by a factor of about three.
It follows that a certain variation in P.sub.abs gives rise to a distinctly smaller corresponding variation in the mechanical characteristic, and this makes it possible to ensure a good stability of the latter, as a function of the said variations in P.sub.abs, and consequently a reproducibility which can be very reliably guaranteed.
If the variations in P.sub.abs are therefore kept within a margin of less than 1%, for example, the corresponding variation in the mechanical characteristic in question will be even better, and this will be virtually equivalent, purely and simply, to keeping the said characteristic at its initial value.